1. Field of the Invention
The present invention relates to an optical semiconductor device including a light receiving element for converting an optical signal to an electric signal and a method for manufacturing the same.
2. Description of Related Art
Light receiving elements convert an optical signal to an electric signal and are used for control sensors. Specifically, the light receiving elements are incorporated in optical pickups for reading/writing signals from/in optical disks, such as DVDs (Digital Versatile Disks) and CDs (Compact Disks).
In recent years, with an increase in data storage capacity and transfer rate, the light receiving elements for use in the optical pickups are required to have high sensitivity to light, reduced noise and enhanced speed.
As the optical disks are becoming denser, the wavelength of a laser light used for reproducing/recording information from/in the optical disks is becoming shorter. So far, an infrared laser of 780 nm wavelength is used for CDs and a red laser of 650 nm wavelength is used for DVDs. Now, development has been proceeding such that a blue laser of about 405 nm wavelength is used for high-density DVDs.
However, the sensitivity of a light receiving element to light is likely to decrease as the laser wavelength becomes short. Suppose that light is absorbed 100% by silicon as material for the light receiving element and completely converted into electricity, sensitivity to light S is proportional to the wavelength of incident light and represented as S=qλ/hc. In the equation, q indicates the quantity of electric charge per electron, λ is the wavelength of incident light (laser light), h is Plank constant and c is light velocity.
Especially in recent years, from the viewpoints of noise reduction and downsizing, there has been required an optical pickup in which a plurality of light receiving elements corresponding to various wavelengths of incident light are integrated on a single semiconductor substrate.
For the purpose of improving the sensitivity to light and reducing the reflectance of incident light at the surface of the light receiving element, a technique of forming an insulating film made of an oxide film and a nitride film as an anti-reflection film on the surface of the light receiving element is generally used as disclosed by Japanese Unexamined Patent Publication No. H10-107312.
(Conventional Technique 1)
Hereinafter, an explanation of an example of a conventional optical semiconductor device including an anti-reflection film on the surface of the light receiving element will be provided with reference to the drawing.
FIG. 14 is a sectional view illustrating a light receiving element including an anti-reflection film which is formed on a diffusion layer in a light receiving region.
A photodiode 11 as an optical semiconductor device includes an N−-type Si substrate 12. A P+-type diffusion layer 13 is formed in the surface of the Si substrate 12 by impurity diffusion as a light receiving region and an anti-reflection film 14 is formed on the P+-type diffusion layer 13.
An electrode 15 for the light receiving region is formed on the Si substrate 12 to be adjacent to the anti-reflection film 14 and an electrode 16 for the substrate is formed to be spaced from the electrode 15. The electrode 15 is electrically connected to a light receiving diffusion layer 13 through a P+-type diffusion region 17 below the light receiving electrode 15. The electrode 16 is electrically connected to the Si substrate 12 through an N+-type diffusion region 18 formed below the electrode 16.
The anti-reflection film 14 is a layered film including a silicon oxide film 19 formed on the light receiving diffusion layer 13 and a silicon nitride film 20 formed on the silicon oxide film 19.
With the anti-reflection film 14 formed on the P+-type diffusion layer 13 (light receiving region), the reflectance of light at the surface of the P+-type diffusion layer 13 approaches 0%. In other words, the ratio of light absorbed into the Si substrate 12 approaches 100%. As a result, light is effectively absorbed into the light receiving region for use in photoelectric conversion while the waste of light at the outside of the light receiving region is reduced. That is, the light receiving element improves in sensitivity to incident light.
According to the conventional technique 1 described above, the obtained device shows a transmittance of at most about 94% when the silicon oxide film 19 is 50 to 90 nm thick, the silicon nitride film 20 is 80 to 100 nm thick and a short wavelength of light, i.e., i rays (λ=365 nm), is incident thereon.
The optimum structure and thickness of the anti-reflection film for minimizing the reflectance of the incident light (maximizing the transmittance) vary depending on the wavelength of the incident light. Therefore, in order to provide an optical pickup corresponding to various wavelengths of light by the conventional technique 1, various OEICs (Opto-Electronic Integrated Circuits) suitable for different wavelengths of incident light, respectively, need to be mounted. For example, the pickup is required to have an OEIC including a light receiving element suitable for a wavelength of 650 nm for DVD and an OEIC including a light receiving element suitable for a wavelength of 405 nm for high-density DVD. However, this is contradictory to the downsizing of the device.
In this respect, for example, Japanese Unexamined Patent Publication No. 2002-118281 proposes a technique of providing an optical pickup in which a single chip corresponds to a plurality of wavelengths of light. An explanation of this technique is provided below.
(Conventional Technique 2)
Hereinafter, with reference to the drawing, an explanation of an optical pickup in which a plurality of light receiving elements are buried in a single substrate at different depths to correspond to different wavelengths of light will be provided with reference to the drawing.
FIG. 15 is a view illustrating the section of light receiving elements incorporated in an optical pickup corresponding to a plurality of wavelengths of light.
According to the conventional technique 2, a Si substrate 40 is prepared by depositing a 10 μm thick low concentration impurity layer 31 on an N−-type Si base substrate 30. The Si substrate 40 is integrated with three light receiving elements, i.e., a light receiving element 41 for CD, a light receiving element 42 for DVD and a light receiving element for high-density DVD.
In the light receiving element 41, an N+-type buried layer 51 is formed at the interface between the N−-type Si base substrate 30 and the low concentration impurity layer 31. The potential of the N+-type buried layer 51 is drawn to the surface of the Si substrate 40 via a contact 61.
In the light receiving element 42, an N+-type buried layer 52 is formed in the Si substrate 40 at a depth of 7 μm from the top surface of the Si substrate 40 (the surface of the low concentration impurity layer 31). The potential of the N+-type buried layer 52 is drawn to the surface of the Si substrate 40 via a contact 62.
In the light receiving element 43, an N+-type buried layer 53 is formed in the Si substrate 40 at a depth of 4 μm from the top surface of the Si substrate 40 (the surface of the low concentration impurity layer 31). The potential of the N+-type buried layer 53 is drawn to the surface of the Si substrate 40 via a contact 63.
Further, P+-type diffusion layers 71, 72 and 73 are formed in parts of the surface of the Si substrate 40 above the N+-type buried layers 51, 52 and 53, respectively.
The light receiving device using the Si substrate 40 shows different absorption coefficients α depending on the wavelength of incident light. Specifically, in order to capture light 81 of wavelength 780 nm for CD with high efficiency, a distance of about 10 μm is required from the substrate surface to the N+-type buried layer 51 because the absorption coefficient α for the light 81 is extremely low. In order to capture light 82 of wavelength 650 nm for DVD, the required distance from the substrate surface to the N+-type buried layer 52 is about 7 μm because the absorption coefficient α for the light 82 is higher than that for the light 81. Further, in order to capture light 83 of wavelength 405 nm for high-density DVD, the required distance from the substrate surface to the N+-type buried layer 53 of about 405 nm is sufficient because the absorption coefficient α for the light 83 is much higher.
In order to use the captured light with efficiency (improve internal quantum efficiency), the depths of the N+-type buried layers 51, 52 and 53 are optimized according to the corresponding wavelengths of light, respectively, thereby reducing loss of light in the Si substrate 40.
As described above, the buried layers are formed in a single substrate in several steps. Specifically, the buried layer for receiving a short wavelength laser light is formed in a shallower position than the position of the buried layer for receiving a long wavelength laser light. Thus, sufficient sensitivity is achieved to various wavelengths of incident light.